Long Discovery ramble.
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Long Discovery ramble.
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Topic: Long Discovery ramble. (Read 4945 times))
Ribbonstone
GTA Senior Contributor
Posts: 12030
Long Discovery ramble.
«
on:
August 07, 2010, 11:59:27 PM »
My 2nd Discovery, hope I learned a little with the first one. After fooling around with some other (much more expensive) rifles, have come to really appreciate the simplicity of the Discovery.
Want to be sure the breech is fully down against the gas tube. Some of the transfer ports are a little long, and tend to raise the front end of the breech a bit (hold it sideways to strong light, looking for a gap). If the transfer port does keep the breech from fitting fully down, would be best to slightly shorten the transfer port, a little at each try, until it fits fully down but still seals.
Worked the trigger over (two screw mod), and it seemed ready to shoot. Previous owner had added the LDC and dual barrel bands, which I’d have done if he hadn’t.
Rifle:
1st test session:
With factory settings,14.3gr. pellets, averaged 21 “good” shots @ 842fps (“good” being within 20fps). That’s seems a little slower than most report, but not far off the average for a pure stock Discovery. Was at the top of its sweet spot at 2000psi and fell off at about 1500psi.
Make a spring guide, a power adjuster screw, and used the full length factory spring. Cranked as far in as would allow the sear to engage, shot the same pellets to 914fps (26.5 foot pounds) but was showing a vel. decline by the 5th shot.
Cut the factory spring by 2 coils and made a “dead end” (dead end probably counts for another ˝ coil). All the way in was back to the factory setting of 840fps and the shot count was about the same (20 good shots).
At the other end of spring tension, backing off to no extra tension at all, rifle gave 32 shots (same pellets…same vel. range limits) @ 745fps with the shortened spring.
The near 100fps difference from max and minimum was more than expected.
Two graphs below. Vel. readings taken at 8 yards (due to light factors).
Going to keep it in the lower graph’s range, about 17.5 foot pounds, and only fill to 1800psi (refilling once the needle dips below 1000psi..or about 30 shots later). I won’t really miss the warm-up shots climbing the curve of a higher pressure fill.
None of it makes a bit of difference if the rifle can’t shoot. Something I’ve found with non-choked barrels is that it’s not that they can’t shoot well but that they are particular about what they will shoot well. Only had 20yards to work with, and enough light for two 5-shot groups per pellet. Just enough testing to weed out the ones that won’t get tested farther.
Early in testing, but it does look like that least two (possibly 3) pellets aren’t going to make the cut for any more testing.
2nd test session:
Decided to try a lighter striker. Can get about the same speeds by reducing the spring tension to the minimum with a stock striker, but found wider variations between individual shots. On a guess, when the striker speed is decreased enough, the variation in friction increases. Was betting that a light striker can move faster, has a little less bearing area, and seems to conserve air by decreasing vale dwell time (the valve closes faster with a lighter striker).
]
Can class vel. variation anyway you’d care to, there don’t seem to be set rules other than “less is better”. Tend to think of speeds +/- 5fps as “great” (that would be 10fps variation), a 20 fps variation as “good”, and a 30fps variation as “marginal”.
This rifle does NOT like the test pellets (Crosman CP’s) all that much but when just getting vel. numbers, they’re pretty consistent. The rifle just won’t group them well. Shooting them up for vel. numbers and saving the more accurate shooting pellets for more serious use is just the cheapskate in me coming out.
3rd test session:
Only change was to move the chronograph to 7 yards to take advantage of natural light (previous test numbers were at 5 yards under artificial light). Same striker settings, same box of pellets.
A little shorter shot string, but the graph would look like the bottom one in the first post. No graph this time, but a pic. of the raw data and some accuracy tests. Targets shot at the pressure level of the sweet spot; in the first test that produced 26 shots +/- 5fps and in the second it produced 25 shots.
Raw data:
Test target (scope adjustments made after each of the first two groups):
Think I’m going to leave it like this for the time being.
4th test session:
Decided to try and stabilize the barrel so the groups would stay more or less at point of aim from day-to-day, also hoped that might make an accuracy difference.
That’s the main point of the Discovery design (as well as the 22XX guns) that really seems 2nd rate is the breech attachment to the bottom tube. The breech is held on by a large screw in the back, a dinky little screw less than 2” ahead of that back screw, leaving over 25” ahead of the dinky breech screw just wiggling however it wants to wiggle. Over tightening that dinky screw is not recommended.
Have noticed that on the Challenger PCP match grade version of the Discovery there is a barrel band right ahead of the breech, which should add some stability and is worth looking into as a modification for a standard Discovery.
Rifle was bought used and already had two barrel bands, one of which was drilled for 4 screws (two to angle down and bear on the gas tube, two angled up for the barrel. I swapped the two around, putting the one with the angled screws just ahead of the fore end.
The standard barrel band was put towards the muzzle. BEHIND the band, a short section of a 1377 plastic spacer was glued to the top of the gas tube. It is not glued to the barrel, just to the gas tube, and is under slight tension when the front band is in place.
(BTW: put it behind the barrel band, as you need to slide the barrel band forward to get it and the breech off when disassembling.)
So the current rifle looks like this:
And the test groups from today look like this:
The first 3 groups (top to bottom on the left) were for adjusting the scope; really didn't change point of impact all that much considering I had the whole top half and the scope off between sight ins.
The best pellets didn’t really get any better, but at least now there are at least 3 pellets that shoot well enough to keep me happy. Non-choked barrels are quite picky about what they will and won’t shoot.
5th test session;
Another day of testing the Discovery .22. Still tuned down where I left it, at the 16-17 foot pound area depending on pellet used.
Been trying, but nothing shoots as well as the RWS H-point, so that’s what it’s going to get even though it reduces range…but considering the accuracy of the stock barrel, keeping the range inside of 40yards is not such a bad thing.
6th test session:
The recrowning/breech smoothing outlined in the below post really can make a difference, and you don’t need big-shot tools to do a good job.
So once I got that all sorted out, you test for pellets. Non choked, cheap, barrels can shoot well, they just don’t usually shoot what you expect them to shoot.
Even though the crown looked pretty good (for a Crosman factory crown) wouldn’t be risking much to recrown. Worst case, and you screw it up, can cut the $25 barrel back 1/3” and try again. For this trial, crowned by hand more or less like outlined in the mentioned post.
Well…it didn’t really help all that much.
In my mind, the “stringing” (groups forming “rips” in one direction or another) decreased. Some pellets shot better, some worse, but over all there was a bit of an improvement.
After reading the posts, decided to give recrowning another try. Really wanted to put the barrel on the lathe and do it “right”, but though a second shot and crowning , this time by hand, might be worth the doing.
Did NOT take the barrel off the rifle, or change any thing else, just the crown (plug the barrel well to keep grit/shavings out of anything important).
Today’s target, post re-re-crowning:
Stringing seems gone, accuracy (at least with the pellet it previously shot well ) is a little improved. Groups have a more round shape, without the stringing noticed earlier. Also notice how far from the last point of impact a new crown can make (call it 4.5MOA).
IF you aren’t comfortable recrowning, then stay away from it. But if you want to give it a try, but are afraid that it would make the rifle shoot worse, don’t worry too much… can always try again.
Tried 30 shots (in the best of the sweet spot for each test) as 6 targets of 5 shots each at 20yards, looking to sort out the best pellet choices.
7th test session:
This is where I left it sighted in 2 weeks ago:
So I started with the same pellets today:
Different target styles, but the POI stayed pretty stable… quite close to where it was left, which was the whole point of all the barrel band fiddling.
So tried the following pellets as well:
Haven’t a good reason, other than barrel vibration/harmonics, for the POI to be generally off to the left with the JSB pellets.
Have been using up some light weight pellets (RWS Hobby) in busting the occasional pest bird. Accuracy hasn’t been great, but as the rang is usually 20-25 yards and these light weight/flat point pellets tend to “smack” with more limited penetration, has been working out just fine. But I won’t stretch the range much farther than 20-25 yards with Hobby pellets as groups tend to grow unexpectedly past that range.
The CP’s were out of the running almost with the first group, but kept on shooting for 30 shots even though it was evident they weren’t going to be in the top 3.
8th test session:
The ding-danged thing shifted again. Found that little breech screw had loosened, which let the @@@ end shift… which changed the point of impact.
Hate that little tiny breech screw and the narrow contact area of the breech to the air tube. Just believe that the breech end of a Disco. might just be its weakest point so far as keeping POI stable.
Decided to try something different. Kept looking at the Challenger’s set up, with a barrel band right ahead of the breech. Don’t have an LW barrel, but if Crosman felt that attachment was worth the doing, thought I’d try it as well (figuring they’d not have bothered if they didn’t see some benefit to it).
Really have no excuse for keeping the rear sight sleeve, other than without it would have that big flat where the 3 set screws fixed it to the barrel and the breech end would have that recess at the front.
So… measure… don’t contact the gauge assembly... cut a section out of the sight sleeve.
Which leaves the front end unsupported, so a second barrel band. This one is one of the set screw versions (all told, 6 threaded holes in that one band).
And that leaves the problem of inletting the stock to take the band. Am NOT a wood worker, so it’s not the closest fit. It’s presentable, but no more than that.
So fully assembled it looks like this:
In past posts had brought the rifle down to the 17foot pound range, with 30-35 good consistent shots. I pushed out of the “sweet spot” a little by shooting 40 shots in testing.
Am keeping that target on my bench and have put away the rifle and won’t touch it for a week. Next weekend will shoot the bottom two rows of diamonds and see if the POI has changed.
(Yes…have tried the CP’s, JSB’s, etc. the rifle evidently can’t read pellet tin labels as it keeps on shooting these H-points better than anything else.)
9th Test session:
Might have gotten a handle on keeping the Discovery pointing in the same direction. Have had an on and off stability issue with the Disco. and have been working on it now and again for months. Checked the height of the transfer port…adjusted the barrel band…used 2 barrel bands…used a custom set screw barrel band…used that custom band and the standard band…tried different spots on the air tube fore each band…etc.
Only been testing for two days, intend to keep testing for the rest of the week. Will shoot on target diamond a day original sight in staying the same, counting only my first 5-shots without warm-up shots. Looking for any shifts in POI.
The first 8 targets are from Sunday.
One target from Monday.
One target from today (Tuesday).
So long as I had the tuned-down .22 Disco out, decided to run a shot string. Had brought this rifle down to 15-16 foot pounds, looking for a good shot count…and for an unregulated, under 2K, tiny-tubed PCP, it’s really not doing badly. A smaller transfer port, much lighter striker, and a mainspring adjustment all aimed at down tuning (and sooner or later, I’ll get it a little lower yet).
Didn’t really plan on this, but it ties in with some of the” dime groups” [posted about on-line over the past few days. Was mostly chronographing and not minding my best bench-rest form, but here’s 50 shots at 20yards:
10th test session:
Week worth of testing:
Been a week. Each day (except Wed….had a staff meeting) would get the rifle out of the safe, bring it to the home test range, and fire 5 shots at a new target diamond. No warm ups, just straight to the target for 5 shots, then put the rifle away until the next day.
Not just a test of the rifle staying put, but the no-warm up groups were probably a good barometer of my work day stress as well.
Whatever else the new barrel band placement does, it does seem to keep the POI in the same place, at least for this test. Will have to mildly bash the rifle around a bit (maybe I’ll take it along in the Jeep to work and back a couple of days) to see how much standard rattling it can stand.
Wish my printer worked…not worthy having to make graph paper… but converted the current shot string into a visual. This is about as good as I can get a Discovery to do. Down tuned to about 17.5 foot pounds and the boxed in area is really a 23fps spread (695 – 717 – 695fps).
11th test session:
For the last 4 days, the gun has ridden in a soft zippered case to and from work every day (27.8 miles round trip) on the floor of a Jeep Wrangler (not the smoothest suspension in the world, and certainly not on the smoothest streets). Made two trips onto pot hole filled construction sites (it’s good to toss a little oil on the front differential now and again by putting it in 4WDhigh).
Today, took the gun out of the Jeep, set up the same target, and fired 5 rounds. It’s the target circle, dated 3/4/10.
Total target:
3/4/10’s target:
2/21/10 target:
Group centers have wiggled around left and right about 1/10” at 20yards, elevation has been nearly dead on. No one can guard against shifting winds, even at 20 yards along a mostly shielded course, so the 1/10th inch “wobble” is not really an indication of the gun shifting.
Think the barrel band just ahead of the breech is working. 5-shot groups at 20yards are under Ľ” long-term (as small as 1/10” and as large as 1/3”). Has been a long trip, lots of testing, spring changes, striker lightening, transfer port strangling, barrel recrowning, etc.
Toned down to a mild 16.5-17 foot pound area, the last 1800 – 975psi shot count ran as followed:
Shots in 10fps = 8
Shots in 20fps = 27
Shots in 25fps = 35
Shots in 30fps =39
So basically, will fill it to 1800, shoot 30 shots if I’m shooting “serious” targets, and then refill. If I’m just shooting at 25-30yards, will stretch that to 40 shots before refilling. Takes about 40 hand pumps to bring it back up to 1800psi,or about one pump per shot.
Think this is as good as I’m going to get.
Even though the power is quite a bit lower than a maxed out Discovery, the longer shot count makes it worth losing a few foot pounds.
In fact, inside of that 34 shots, there are 25 consecutive shots that don’t vary by more than 10fps.
Accuracy has been pretty static over these tests, have managed to keep it shooting to about the same point of impact (these targets were shot where I left the rifle’s scope settings nearly a month ago) but find it hard to beat Ľ” for 5 5-shot groups at 20yards.
Crosman still has the best shooting $25 barrels I’ve ever used. Those barrels are picky about what they will shoot, and may need a recrown, but so far have always found something they shoot well.
«
Last Edit: August 08, 2010, 12:06:01 AM by Ribbonstone
»
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Gary
Sharp Shooter
Posts: 624
Re: Long Discovery ramble.
«
Reply #1 on:
August 08, 2010, 03:10:20 AM »
Great job with the write up. I have done some of the mods you mentioned , power adjuster,two coils off the striker spring, lightened the striker,recrowned barrel,three screw mod on trigger(didn't care for that one. I'm sure the sear was dragging on the striker),and a new transfer port. Started small with the hole .120 dia. and increased in size up to .128 dia. Now getting 40 good shots at 750 fps. Fill to 2K and refill at 1k. My rifle seems to like the Crosman Premiers and JSB's both14.3 grs. It will shoot other pellets ok but really likes those two.
Gary
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Rocker1
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Real Name: David
Re: Long Discovery ramble.
«
Reply #2 on:
August 08, 2010, 06:19:14 AM »
wow!!! great write up, thanks guys like you who take the time to show your experimentation is what gives this forum a great knowledge base. thanks david
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dk1677
Squirrel !
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Re: Long Discovery ramble.
«
Reply #3 on:
August 08, 2010, 08:29:59 AM »
Great work! Ditto to what David said!
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DaveShooter
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Re: Long Discovery ramble.
«
Reply #4 on:
August 09, 2010, 11:31:49 AM »
Man that rifle is rock'en. You did a great job tricking that puppy out. Nice groups and great job. Give yourself a pat on the back. My disco is still just a stock rifle. It like the 14.5 RWS pointed pellets. After seeing what or how tight ya got your baby to shoot all I can say is WOW!!!!!
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tworr
Plinker
Posts: 294
Re: Long Discovery ramble.
«
Reply #5 on:
August 09, 2010, 11:48:21 AM »
Excellent write up!! I enjoyed it very much.
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Ribbonstone
GTA Senior Contributor
Posts: 12030
Re: Long Discovery ramble.
«
Reply #6 on:
August 09, 2010, 08:09:20 PM »
has been in the safe too long, so took it out for a shot string test. Had last set it to lower power for more shots...basically, this power level is nough for the game I hunt and the distance it's hunted, so why not set it this way?
.22 Discovery 14gr. RWS Match
(Lighting changed, am going to have to assume the “err” shots (error) are pretty close to the shots before and after.)
130BAR
759
757
759
762
770
772
760
769
770
err
765
767
766
765
766
778
776
778
777
780
779
err
777
775
780
779
785
782
err
778
785
776
777
771
772
771
774
763
760
80 BAR
750
755
744
716
755-785-755 (30fps) = 39 shots
765-785-765(20fps) = 30 shots
775-785-775(10fps) = 18 shots 780fps/ 14gr = 18.9 foot pounds
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Springer25
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Re: Long Discovery ramble.
«
Reply #7 on:
August 09, 2010, 11:32:50 PM »
Wow that is quite the write up. very nicely done. your getting very good groups with that thing.
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Bryan21184
Plinker
Posts: 205
Re: Long Discovery ramble.
«
Reply #8 on:
December 21, 2011, 01:17:43 AM »
Wow very good info
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0351_Vet
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Re: Long Discovery ramble.
«
Reply #9 on:
December 21, 2011, 01:22:19 AM »
Quote from: Bryan21184 on December 21, 2011, 01:17:43 AM
Wow very good info
Yah, not bad for 16 months old.....good posts are like fine wine....
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SquirrelSniper25
Trevor
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Re: Long Discovery ramble.
«
Reply #10 on:
December 21, 2011, 12:12:03 PM »
Wow that gun/shooter accurate!
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Long Discovery ramble.